We have obtained data from several experiments suggesting that the anterior pituitary contains multiple dopamine receptors. First a biphasic Scathard plot was observed for 3H-spiroperidol binding, secondly GTP caused a shift in binding affinity of the agonists dopamine and apomorphine but not bromergocryptine, and lastly low concentrations of dopamine stimulated prolactin release while higher concentrations inhibited prolactin release. The stimulation of prolactin might be related to a D1 dopamine receptor while the inhibition is mediated by a D2 receptor. Destruction of tuberoinfundibular dopaminergic neurons results in an increased responsiveness of the anterior pituitary to dopaminergic inhibition of prolactin release both in vivo and in vitro. We have extended these findings to show that the ability of dopamine to inhibit prolactin synthesis is also potentiated. 10 to the minus 8th power M dopamine significantly suppressed prolactin synthesis from anterior pituitaries of lesioned rats incubated with 3H-leucine but not from control anterior pituitaries. Hypothalamic lesions also caused a 2 fold increase in the density of mammotrophs as determined by immunocytochemical staining and morphometric analysis. The number of dopamine receptors as estimated by 3H-spiroperidol binding was significantly reduced in the pituitaries of lesioned rats. This change is difficult to reconcile with increased responsiveness to dopaminergic suppression of prolactin synthesis and release. Estrogen treatment was found to have no effect on the number or affinity of anterior pituitary dopamine receptors.